Abstract:

Not much is known about the nitrogen (N) uptake capacity and N-form preference of tropical trees. In a
replicated labelling experiment with 15N-ammonium, 15N-nitrate and dual-labelled glycine applied to saplings of six
tree species from southern Ecuadorianmontane forests, we tested the hypotheses that (1) the saplings of tropical trees
are capable of using organicNeven though they are forming arbuscularmycorrhizas, and (2) with increasing altitude,
tree saplings increasingly prefer ammonium and glycine over nitrate due to reduced nitrification and growing humus
accumulation. Three- to 5-y-old saplings of two species each from 1000, 2000 and 3000 m asl were grown in pots
inside the forest at their origin and labelled with non-fertilizing amounts of the three N forms; 15N enrichment was
detected 5 days after labelling in fine roots, coarse roots, shoots and leaves. The six species differed with respect to
their N-form preference, but neither the abundance of ammonium and nitrate in the soil nor altitude (1000–3000
m asl) seemed to influence the preference. Two species (those with highest growth rate) preferred NH4+ over NO3?,
while the other four species took up NO3? and NH4+ at similar rates when both N forms were equally available. After
13C-glycine addition, 13C was significantly accumulated in the biomass of three species (all species with exclusively
AM symbionts) but a convincing proof of the uptake of intact glycine molecules by these tropical montane forest trees
was not obtained.